The Boat Safety Scheme and good practice requires the use of multi-strand cables.

These are sized by a two-part number showing:

So a piece of cable sized 28/0.30 has 28 strands, each strand being 0.30mm in diameter.

You should also be given the cross sectional area of the conductor (all the strands together) and the maximum current rating for the cable (un-loomed/un-bundled)

Ideally each strand should be tinned to minimise corrosion as damp is drawn down between wire & insulation. However inland boats can "get away" with non-tinned cable.

Boats built under/to the Recreational Craft Directive are required to have cables with a minimum number of strands, other boats are not required to adhere to this standard. This only applies to the thinnest cables, so choosing a slightly thicker one avoids this.

Most easily available cables have PVC insulation. This has a relatively low resistance to heat, and as electricity flowing through a cable causes heat the current flow has to be "de-rated" if the cable is tightly bundled/"loomed up" with other cables so the heat cannot escape so well. If you over-specify your cables you will minimise this problem. In most cases the Volt-drop calculation (see later) will ensure this is not a problem.

There are other insulation materials that allow a higher working temperature (Tri-rated cables, silicone rubber etc) that also minimise any potential problem.

Any cable has some resistance to current flow. The resistance turns some of the voltage to heat so the longer the cable the more voltage is lost to heat.

Ideally the maximum voltdrop on a cable run should be about 0.3 volts, however in most general applications 0.5 volt should have no marked effect.

Take particular care to minimise voltdrop on:-

For runs of much over 2 or 3 metres ignore any current capacity marked on a drum or in a table/catalogue apart from taking it as a maximum – CALCULATE THE VOLTDROP.

A tunnel light on a 16.5m boat uses a 55-watt bulb – what size conductor is needed?

This means that the lamp would only receive 10.8 volts, so it would be a bit dim – but probably not to any noticeable extent.

As tunnel lights are only on for a relatively short period, there is no regulation governing them, and 55 watts is far too bright for oncoming boaters anyway, 28/0.30 would do the job.

If it was a yacht’s mast head lamp one would have to take a less cavalier approach to cable sizing because of international regulations regarding the boat’s statutory lights.

One can also look up the voltdrop per amp per meter on a table and do a simple multiplication.

The difference is due to rounding errors when compiling the table, but note the errors lead you to specify thicker cable.

Once you have done a few key calculations you will find that you know what size of cable needs to be used.

For instance Shoreline say use 1 mm² cable for each metre of run between battery and fridge. Use this sized cable for both the out and return run. They also say fuse at about 3 amps. The two figures just do not make sense unless you know a bit about the way compressor fridges start up and their "inbuilt protection circuits". Do not doubt the makers – they are trying to ensure you get a reliable product.

I suspect similar "rules" exist for inverters!

When using vehicle grade PVC cables with 0.30 conductors, simply halve the number of strands to get a safe maximum current rating (this takes no account of voltdrop). This does not work for cables larger than 120/0.30

Most boats tend to be in a state of continuous "improvement", so if you fit over large cables in the first place, you can add some extra "branches" with simply a change of fuse or circuit breaker. This saves a lot of problems in trying to run a whole new circuit back to the battery or busbars.

You must keep a note of the cable size you have used, so you can ensure your additions do not overload the cable – fire is nasty stuff, especially when twinned with thick, black PVC smoke!

Fuses are not very accurate with their rating, so can blow at rather different loads than that stated.

The fuse wire does heat up and cool down during use, so, over a long period of time they blow because the wire has deteriorated due to the constant thermal stress.

The fuse wire must cause voltdrop.

They are usually cheap to replace and can be "jury rigged" on a "get you home" basis – once you understand the dangers of what you are doing.

Do not use the ceramic "pointy end" vehicle fuses. They always seem to develop poor contact at their ends.

Should not cause significant voltdrop (they use magnetism/heat to operate).

Can be easily reset if they trip accidentally.

Should not deteriorate over time.

Circuit breakers (MCB = miniature circuit breaker) are probably the most satisfactory circuit protection method BUT:-

The correct selection of circuit breakers involves studying the tripping characteristic graph for each breaker, however for an amateur simply specify the breaker/fuse for the maximum current rating of the cable. You would have increased this by 20% for inductive loads, so this should be fine.

For simplicity this section covers both fuses and circuit breakers.

It is unlikely that a fuse or circuit breaker will be available that exactly matches your load so:-

Select the next size fuse/circuit breaker above the capacity of the cabel if there is not an exact match.

If having done this you find something is constantly blowing/tripping the protection:

The idea is to fuse very critical, high current things individually and combine low current or items that are unlikely to be used together.

The interior lights are split into port and starboard, so you always have some light to fault find by!

Ensure that the fuse/circuit breaker is rated no higher than the maximum cable rating.

If any cables are to be run behind fixed panels, then run them through trunking (available at DIY shops and electrical outlets).

Where a switch, socket, light etc is to be fitted through a fixed panel terminate the trunking with a suitable patress box (the thing you have behind your switches and sockets at home). Also fit patress boxes behind the mounting point for any ceiling lights – these will give you space to accommodate the "choc-block" connectors these lights often use to connect to the permanent wiring.

Only loom (bind up) cables in the engine room to help protect them from damage – use non-sticky PVC tape so they remain flexible – ordinary insulating tape sticks all the cables together and makes the loom stiff and difficult to bend.

Loosely bind cables with "zip ties" every few feet and support them every 300mm if they are not to be run in plastic/metal trunking. This makes the bundle easy to open if you ever get a cable burn out.

Always install extra cables through trunking, ensuring you fit more trunking if the wires are starting to be a tight fit. This will allow you to "pull through" any cables in the future, or even use the extra cable in future extensions (as long as its large enough).

Keep mains wiring at least 30mm away from the low voltage DC cables. If they have to be closer make sure at least one of them is in trunking. This is to minimise the risk of a fault putting mains voltage onto your DC circuits – with fatal results.

Keep electronic equipment (navigation equipment) separate from both the mains and DC circuits. Also only cross such circuits at right angles. This will ensure that any interference from pulsating magnetic fields is kept to a minimum.

In some way identify the ends of each cable & mark it on a list or diagram. Rubber or crimp letter/number bands are available or even a white insulation tape "flag" marked with permanent marker would do.

You can wire things like the lighting and 12 outlet circuits up individually and only join their conductors at the fuse & busbar as shown:

Ideally there will be a second, higher rated fuse/circuit breaker closer to the battery to protect the circuits if the "main fuse" failed to operate. There is often not.

Some circuit breaker boxes fit a master circuit breaker, or you could fit a very large capacity fuse in the main + battery to busbar cable, close to the battery.